As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semi-conductor wafers. The basis for these techniques is the notion that a subject may be examined by analyzing the reflected energy that results when a probe beam is directed at the subject. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in magnitude of the reflected intensities are analyzed. Scatterometry is a related technique that measures the diffraction (optical scattering) that the subject imparts to the probe beam.
Techniques of this type may be used to analyze a wide range of attributes. This includes film properties such as thickness, crystallinity, composition and refractive index. Typically, measurements of this type are made using reflectometry or ellipsometry as described more fully in U.S. Pat. Nos. 5,910,842 and 5,798,837 both of which are incorporated in this document by reference. Critical dimensions (CD) including line spacing, line width, wall depth, and wall profiles are another type of attributes that may be analyzed. Measurements of this type may be obtained using monochromatic scatterometry as described in U.S. Pat. Nos. 4,710,642 and 5,164,790 (McNeil). Another approach is to use broadband light to perform multiple wavelength spectroscopic reflectometry measurements. Examples of this approach are found in U.S. Pat. No. 5,607,800 (Ziger); U.S. Pat. No. 5,867,276 (McNeil) and U.S. Pat. No. 5,963,329 (Conrad) (each of the patents is incorporated in this document by reference). Still other tools utilize spectroscopic ellipsometric measurement. Examples of such tools can be found in U.S. Pat. No. 5,739,909 (Blayo) and U.S. Pat. No. 6,483,580 (Xu). Each of these patents and publications are incorporated herein by reference.
Photo-modulated reflectance (PMR) is another technique used to perform non-destructive inspection and analysis of semi-conductor wafers. As described in U.S. Pat. No. 4,579,463 (incorporated in this document by reference), PMR-type systems use a combination of two separate optical beams. The first of these, referred to as the pump beam is created by switching a laser on and off. The pump beam is projected against the surface of a subject causing localized heating of the subject. As the pump laser is switched, the localized heating (and subsequent cooling) creates a train of thermal waves in the subject. The second optical beam, referred to as the probe beam is directed at a portion of the subject that is illuminated by the pump laser. The thermal waves within the subject alter the reflectivity of the subject and, in turn, the intensity of the reflected probe beam. A detector synchronously samples the reflected probe beam synchronously with the switching frequency of the pump laser. The resulting output is used to evaluate parameters such as film thickness and material composition.
The article “Applications of Optical Beam-Induced Reflectance Scans in Silicon Processing” (Gary E. Carver and John D. Michalski, IEEE Journal of Quantum Electronics, Vol. 25, No. 5 1989) discloses a second type of optical metrology system that uses a laser-generated pump beam. For this second system, an off-axis probe beam intersects the subject at a relatively large angle of incidence. The pulsed pump is directed normally to the subject and modulates the intensity of the reflected probe beam. The modulated intensities are used to evaluate the subject. The authors have reported that this combination results in an enhanced sensitivity to near-surface electrically active defects.
As the geometries used in semiconductors continue to decrease, optical metrology tools are forced to analyze smaller and smaller structures. For most optical metrology systems, this means using smaller measurement spots (the area within a subject that the detected light originates from during measurement). At the same time, it is not always practical to reduce measurement size, particularly for ellipsometers. This is partially because ellipsometers are typically configured to operate at non-normal angles of incidence (unlike reflectometers and the some of the PMR-type systems described above). The non-normal angle of incidence increases sensitivity to thin-film properties. At the same time, non-normal incidence elongates the measurement spot by a factor equal to 1/cos(θ) where θ is the angle of incidence. For an incident angle of seventy-degrees, for example, this elongation means that the measurement spot is spread to nearly three times its normal length.
Chromatic aberration is a second obstacle that often limits reductions in measurement spot sizes for ellipsometers. Chromatic aberration results when an optical system transports light in a wavelength dependent fashion. In spectral ellipsometers, the probe beam includes a range of wavelengths and chromatic aberration tends to create different measurement spot sizes for the different probe beam wavelengths. This is particularly true for spectral ellipsometers that use diffractive optical elements. The overall result is that the minimum size of the measurement spot is influenced by the range of wavelengths included in the probe beam and the amount of chromatic aberration present of the spectral ellipsometer.
One approach for reducing measurement spot sizes in ellipsometers is to use high numerical aperture lenses to perform measurement spot imaging. This is described, for example, in U.S. Pat. No. 5,596,411 (incorporated in this document by reference). The use of the high numerical aperture lens increases the accuracy with which the measurement spot may be imaged. The high numerical lens also creates a spread of angles of incidence all converging on a relatively small illumination spot. For some applications, the multiple angle of incidence approach provides an enhanced ability to deduce properties of the sample being analyzed. At the same time, the use of multiple angles of incidence increases the difficulty (i.e., computational complexity) of interpreting the resulting measurements. In some cases, this can make this particular approach impractical.
A second approach for reducing measurement spot sizes in ellipsometers is described in U.S. patent application Ser. No. 10/319,189, filed Mar. 13, 2002 (incorporated in this document by reference). For this approach, a shallow (or near normal) angle of incidence is used to produce a relatively small measurement spot size. In combination with the shallow angle of incidence, a rotating compensator is used to impart a wavelength dependent phase delay to the probe beam. A detector translates the reflected probe beam into a signal that includes DC, 2ω and 4ω signal components (where ω is the angular velocity of the rotating compensator). A processor analyzes the signal using the DC, 2ω and 4ω components. The use of the DC component allows thin film characteristics to be accurately analyzed without the need for larger angles of incidence. At the same time, the use of normal or near normal incidence has a tendency to reduce or eliminate the distinction between p and s polarized light and may make this approach unsuitable for some applications.
Based on the preceding description, it is clear that there is a continual need to produce ellipsometers that operate using smaller and smaller measurement spots. This need is particularly true for semiconductor manufacturing where structure sizes continue to decrease.